1. Field of the Invention
This invention relates to a vehicle, particularly to a vehicle equipped with an inverter to drive a motor for driving the vehicle and a control device thereof.
2. Description of the Background Art
In recent years, an eco-friendly vehicle to use a motor for driving the vehicle such as an electric vehicle, a hybrid vehicle and a fuel cell vehicle is drawing public attention.
Japanese Patent Laying-Open No. 09-070195 discloses a control device of a motor capable of handling even in a case where rapid heat generation is caused in a switching device. In a case where a rotation speed of the motor is sufficiently low, this motor control device determines that the motor is locked by external force and switches carrier frequency of a PWM (pulse width modulation) signal from a normal value of 10 kHz to 1.25 kHz. In accordance with a decrease in the carrier frequency of the PWM signal, switching frequency of the switching device of an inverter is decreased and a switching loss is reduced. Therefore, there is no risk that rapid heat generation is caused in the each switching device of the inverter even when the motor is locked.
In the inverter for driving the vehicle, there are strong demands for downsizing and reducing cost. Therefore, it is not possible to sufficiently increase carrier frequency fc relative to output frequency fv in order to suppress the switching loss. Originally, the pulse number in PWM control is desirably about 15 pulses or more per one cycle of output frequency fv. Particularly when high torque is generated, an inverter conduction loss is increased and a heat generation amount is also increased. Therefore, there is a need for preventing overheat and reducing the heat generation in this region. However, when the inverter is designed so as to drive with high carrier frequency in all operating regions, size of the inverter becomes bigger and the cost of the inverter is increased.
When carrier frequency fc is simply decreased in order to reduce the switching loss, a controllability of the motor is deteriorated. Therefore, since there is a risk of vibration of the vehicle due to pulsation of the torque and breakage of parts due to out-of-control on an electric current, it is not allowable to simply decrease carrier frequency fc. Synchronous PWM control is sometimes adapted in order to ensure the controllability at the low pulse number. However, conversely at the time of low torque such as steady traveling with little noise inside the vehicle, there is a possibility that an electromagnetic sound by inverter switching is annoying. Although frequency of the sound can be changed by changing the pulse number, it is not possible to arbitrarily change the frequency due to restriction of synchronization.
Losses in the inverter and the motor are changed by carrier frequency fc so as to influence over efficiency of the inverter and motor. Therefore, even with an operating condition without problem in the controllability and the noise, there is a need for properly determining carrier frequency fc. Particularly for a purpose of the vehicle, when the efficiency is improved in a practical region with relatively low torque highly frequently used, fuel consumption is improved.
Since there is a subject of achieving ensuring of the controllability, reduction of the cost, reduction of the noise inside the vehicle and further improvement of the efficiency at the same time, an inverter control method suitable for a way of using the vehicle is desired.
Here, as shown in Japanese Patent Laying-Open No. 2006-217776, in a case where the synchronous PWM control is adapted, in order to prevent distortion of even harmonics from overlapping with output voltage, carrier frequency fc is selected to be an odd multiple of output frequency fv or multiples of three of the carrier frequency in order to match the output voltage of a three-phase inverter; and two kinds of carriers which are inversion and non-inversion are provided. Therefore, the carrier frequency fc has only discrete value.
For example, a case where there is a restriction that carrier frequency fc is sixfold, ninefold, twelvefold, fifteenfold . . . of output frequency fv will be described. Hereinafter, the term pulse number will be used in the description, e.g. when the carrier frequency is sixfold of output frequency fv, the pulse number is six pulses. In order to distribute carrier frequency fc, high carrier frequency fc is used such as nine pulses of 1.5 times more or twelve pulses of 2 times more than minimum-required six pulses. Therefore, the switching loss is increased, a problem of overheat in the inverter is caused, and hence there is a cost problem in order to prepare an inverter resistant to the above problem.
A method of selecting carrier frequency fc in an operating state capable of achieving the controllability and the reduction of the noise at the same time is not clearly stated. Therefore, in a case where carrier frequency fc is set to be higher than necessary, there is a possibility of an increase in the loss and a deterioration of the fuel consumption. In other words, since too much importance is conventionally given to improvement of the controllability and the reduction of the noise, there is sometimes a case where the cost of the inverter is high and there is some room for improving the fuel consumption.